Extreme ultraviolet (EUV) lithography systems use an operating wavelength of 13.5 nm to project pattern information from an EUV-reflective photomask onto a semiconductor wafer at reduced magnification. The stringent quality standards required for EUV photomasks can be achieved with actinic mask inspection and metrology tools, which use the same 13.5-nm wavelength for microscopy. Compared to DUV (193-nm deep ultraviolet) or e-beam imaging, an actinic system is more sensitive to mask characteristics that impact EUV lithography performance. Also, an actinic system can view the mask through a protective pellicle (cover plate), which is transparent to EUV but not to DUV or e-beam.
One such actinic inspection system is the EUV Aerial Image Measurement System (AIMS) made by Carl Zeiss, Inc. [Ref. 1]. This is a full-field imaging microscope, which uses illumination and projection optics similar to EUV lithography systems but operating at a lower numerical aperture. The need for wide-field, diffraction-limited imaging and high throughput makes the AIMS a very expensive system.
The Center for X-ray Optics (CXRO) at Lawrence Berkeley National Laboratory, and Samsung Electronics Co., Ltd., have demonstrated a relatively simple and low-cost EUV mask metrology system, which scans a single, diffraction-limited EUV focal spot across the mask while the mask reflectance signal is recorded [Ref 2]. The system uses a tabletop, high-harmonic-generation (HHG) EUV source and a Fresnel zoneplate focusing optic. But with a single scanning point and very low source power, the system is too slow for commercial photomask qualification.
CXRO has also developed a higher-end EUV microscopy tool, the “Semiconductor High-NA Actinic Reticle Review Project” (SHARP). [Ref 3] It is a low-throughput research system using a synchrotron EUV source, which would be impractical for commercial use.
U.S. Pat. No. 9,188,874 (“Spot-Array Imaging System for Maskless Lithography and Parallel Confocal Microscopy”, hereafter the '874 patent) and U.S. Pat. No. 9,097,983 (“Scanned-Spot-Array EUV Lithography System”, hereafter the '983 patent), and other prior art cited therein, disclose massively-parallel spot-scanning systems that overcome the throughput limitations of single-spot scanners such as the CXRO/Samsung system. The EUV focusing elements can be achromatic phase-Fresnel microlenses (Schupmann doublets), which allow the use of a laser-produced plasma (LPP) EUV source. (An LPP source has much more power than HHG sources and is much less costly than synchrotron sources, but its wide spectral bandwidth necessitates achromatic focusing lenses.) The microlenses can be constructed to offset and nullify geometric aberrations in the illumination optics, so the optical system could be simpler and less costly than an AIMS-type system.
The prior-art microscopy systems outlined above typically produce reflectance amplitude images of an inspection surface, but provide little or no information on image phase. Techniques such as Zernike phase-contrast microscopy can be used to provide phase sensitivity, but at the expense of reduced amplitude sensitivity. [Ref. 4] An alternative imaging technique, Coherent Diffraction Imaging (CDI), can simultaneously determine reflectance amplitude and phase by measuring the inspection surface's far-field, angular reflectance distribution as the sample is translated across the illumination beam. [Ref. 5] This is a “lensless” imaging technique, which can produce diffraction-limited images without projection optics. But CDI has several limitations: It requires a highly coherent EUV source such as a synchrotron or HHG source. Complex numerical algorithms are required to compute the image from the measured angular reflectance distribution. For EUV mask inspection, defect sensitivity may be limited because only a small portion of the detector signal originates from the defect area.